Re: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item

[zhouchengming]

On 2016/5/6 5:57, Andrea Arcangeli wrote:
> Hello Zhou,
>
> Great catch.
>
> On Thu, May 05, 2016 at 08:42:56PM +0800, Zhou Chengming wrote:
> >   	remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
> > +	up_read(&mm->mmap_sem);
> >
> >   	spin_lock(&ksm_mmlist_lock);
> >   	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
> > @@ -1666,16 +1667,12 @@ next_mm:
> >   		 */
> >   		hash_del(&slot->link);
> >   		list_del(&slot->mm_list);
> > -		spin_unlock(&ksm_mmlist_lock);
> >
> >   		free_mm_slot(slot);
> >   		clear_bit(MMF_VM_MERGEABLE,&mm->flags);
> > -		up_read(&mm->mmap_sem);
> >   		mmdrop(mm);
>
> I thought the mmap_sem for reading prevented a race of the above
> clear_bit against a concurrent madvise(MADV_MERGEABLE) which takes the
> mmap_sem for writing. After this change can't __ksm_enter run
> concurrently with the clear_bit above introducing a different SMP race
> condition?

Yes, I didn't notice this problem... Thanks.
> > -	} else {
> > -		spin_unlock(&ksm_mmlist_lock);
> > -		up_read(&mm->mmap_sem);
>
> The strict obviously safe fix is just to invert the above two,
> up_read; spin_unlock.
>
> Then I found another instance of this same SMP race condition in
> unmerge_and_remove_all_rmap_items() that you didn't fix.
>
> Actually for the other instance of the bug the implementation above
> that releases the mmap_sem early sounds safe, because it's a
> ksm_text_exit that takes the clear_bit path, not just the fact we
> didn't find a vma with VM_MERGEABLE set and we garbage collect the
> mm_slot, while the "mm" may still alive. In the other case the "mm"
> isn't alive anymore so the race with MADV_MERGEABLE shouldn't be
> possible to materialize.
>
> Could you fix it by just inverting the up_read/spin_unlock order, in
> the place you patched, and add this comment:
>
> 	} else {
> 		/*
> 		 * up_read(&mm->mmap_sem) first because after
> 		 * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
> 		 * already have been freed under us by __ksm_exit()
> 		 * because the "mm_slot" is still hashed and
> 		 * ksm_scan.mm_slot doesn't point to it anymore.
> 		 */
> 		up_read(&mm->mmap_sem);
> 		spin_unlock(&ksm_mmlist_lock);
> 	}
>
> And in unmerge_and_remove_all_rmap_items() same thing, except there
> you can apply your up_read() early and you can just drop the "else"
> clause.
>
> .

Your change is better and the comment is good and clear.
So I will send a PATCH v2 based on your suggestion. Thanks.